Taste responses of cortical neurons

Branches of the facial, glossopharyngeal and vagus nerves which synapse with the receptor cells in the taste buds convey taste messages to the rostral part of the nucleus of the solitary tract. The second relay nucleus for ascending taste input is the parabrachial nucleus of the pons. The third relay nucleus is the medial parvocellular component of the ventrobasal complex of the thalamus. This thalamic nucleus projects to the cortical taste area. Another ascending projection site of the parabrachial nucleus is to the lateral hypothalamus, amygdala and bed nucleus of the stria terminalis. In monkeys, neurons from the gustatory area of the solitary nucleus directly reach the thalamic taste area, bypassing the parabrachial nucleus. Taste-elicited reflex activities are based on a hedonic (acceptable or rejective) aspect of taste, and are basically determined in the brain stem without activation of cortical neurons. The cerebral cortical taste area is located dorsal to the rhinal sulcus in or near the insular cortex in different species of animals. Besides this taste area, taste inputs also project to the tongue tactile area of the SI in monkeys, cats and rats. Taste-responsive neurons in SI area are also responsive to light tactile stimulation of the tongue. Human clinical case reports suggest that discrimination or recognition of taste quality are processed in the cortical taste area. In animal experiments, it is difficult to determine the functional significance of this area. However, recent behavioral studies using the conditioned taste aversion technique in rats suggest that the cortical taste area plays an important role in cognitive (learning, memorial, associative and discriminative) processes of taste sensation. Summated cortical evoked potentials have been recorded in human and rats to electrical and taste stimulations applied to the tongue surface. Recording of gustatory primary evoked potentials is unsuccessful in human subjects. The responses to taste stimulation are composed of an early component, which is induced by mechanical stimulation of a test solution poured on the tongue surface and a slow component, which is assumed to be the gustatory response. Essentially similar results are obtained for cortical summated responses to taste stimuli in rats. Besides these averaged evoked potentials, arousal changes of EEG occur in response to taste stimulation. There is a possibility that the arousal response can be used an objective indicator for intensity and hedonics of perceived taste sensation.(ABSTRACT TRUNCATED AT 400 WORDS)